The premises of your question are wrong.
Lets start with the title. "Legalising dangerous drugs". The danger of marijuana is debatable. Alcohol and Tobacco are equally if not more dangerous (as I will show later).
So it puts the people at risk of facing drugs even against their will.
How so? I've never had to take any drugs -- including Tobacco and Alcohol -- against my will. And again, you are already facing drugs like Alcohol, Tobacco, caffeine and pain pills on a daily basis. Abuse of pain pills is quite common in the US.
For example, there's possibility for children to accidentally come across drugs.
And guns, especially in the US. And bleach, and pesticides and tobacco, alcohol and caffeine. Yet none of these put children in undue danger if the parents keep an eye on them and educate them on the dangers of unknown substances.
It may undermine the nation's gene pool.
Not more than alcohol, tobacco and guns. Probably even less.
All of your points pre-suppose that drug-use will rise. That is not based in fact:
In five states that decriminalized marijuana between 2007 and 2015, there was no corresponding rise in the drug’s use among young people, a new analysis shows.
So if there is no rise, none of your previous points carries any water, anyways.
Thus, my question is as follows: What is the point of legalising such dangerous drugs? Do disadvantages not outweigh advantages at this point?
The disadvantages are small to none, while there are great advantages:
The researchers found that decriminalization was associated with a 75 percent reduction in marijuana-related arrests of people under the age of 21. Meanwhile, there was no increase in reported use by the high school students who took the survey.
Less arrests are somewhat to be expected. Why is that a good thing, though?
“An arrest can have a long-term impact on a teenager, even if that individual isn’t ultimately found guilty or sent to jail. Scholarship opportunities and grants can be lost, and in some states, drivers’ licenses are confiscated. There are several important life consequences that go along with having a criminal record after an arrest for marijuana.”
The negative consequences associated with drug arrests are a primary reason a number of public health organizations, including the American Academy of Pediatrics, have announced support for decriminalization while still opposing pot legalization.
"Decriminalizing pot doesn’t lead to increased use by young people", Washington University in St. Louis, Jim Dryden, July 2018
Decriminalization of drugs is no recent development, either. In the US, it started in 1973.
1973: Oregon became the first state to decriminalize cannabis – reducing the penalty for up to one ounce to a $100 fine.
1975: Alaska, Maine, Colorado, California, and Ohio decriminalized cannabis.
A 1989 study already found various positive effects:
The available evidence indicates that the "decriminalization" of marijuana possession had little or no impact on rates of use. Although rates of marijuana use increased in those U.S. states which reduced maximum penalties for possession to a fine, the prevalence of use increased at similar or higher rates in those states which retained more severe penalties. There were also no discernable impacts on the health care systems. On the other hand, the so-called "decriminalization" measures did result in substantial savings in the criminal justice system.
So there is no rise in numbers, but positive effects on the healthcare system and justice system.
So how dangerous is cannabis?
In most cases, drinking alcohol is not life-threatening. However, when people consume too much alcohol, it can be fatal. The CDC reports that nearly 88,000 alcohol-related deaths occur each year. And binge drinking accounted for about half of these deaths.
In comparison, the number of deaths caused by marijuana is almost zero. A study found that a fatal dose of TCH, the potent chemical in marijuana, would be between 15 and 70 grams. To give you an idea of how much marijuana that is, consider that a typical joint contains about half a gram of marijuana. That means that you would have to smoke between 238 and 1,113 joints in a day to overdose on marijuana. That’s a lot of joints.
So overdosing on marijuana is a lot harder than on alcohol.
When it comes to what substance will put someone at risk for getting hurt or hurting others, alcohol is considered to cause the most harm.
A study on marijuana use and intimate partner violence found that couples who used marijuana had lower rates of intimate partner violence in the first 9 years of marriage. In fact, men who used marijuana were the least likely to commit an act of intimate partner violence against a spouse.
Less domestic violence when marijuana is involved compared to when alcohol is involved. I'd say that is a good thing.
Besides alcohol, marijuana is the most commonly detected drug in drivers involved in car accidents. One study found that marijuana increased the odds of being in car accident by 83%.
You may think that 83% is high, but when alcohol was involved, the odds of being in a car accident increased more than 2,200%!
To state the obvious: nobody should drink and drive or get high and drive. The sad truth is, people do. And if I look at the above numbers, it seems legalizing marijuana isn't the problem, its continuing to tolerate alcohol.
"The Great Debate: Alcohol Vs Marijuana", Lauren Villa, MPH, American Addiction Centers, 2020
So the gist is: Marijuana isn't as dangerous as you obviously think it is, and the negative consequences you fear have no basis in fact. In fact, studys show the very opposite: No increase in usage, but less crime, its better for the healthcare system and for the justice system. So the advantages seem to outweigh the disadvantages.